Sumitomo Chemical Co., Ltd.
Process & Production Technology Center
Evaluation and Prevention of
Electrostatic Hazards in
Chemical Plants
Kiyoshi O TA
Nowadays, electrostatic theories are usefully applied to various industries. On the other hand static electricity
cause ESD (electrostatic discharge) problem in high technology industries or fire and explosion in chemical
industries. In order to prevent incidents caused by electrostatic discharge, it is important that operators or staffs
working in chemical plants find electrostatic potential hazards and have consultations with safety experts. Electrostatic charge and discharge phenomena, countermeasures against static electricity and some methods for evaluating electrostatic hazards are described in this paper.
This paper is translated from R&D Report, “SUMITOMO KAGAKU”, vol. 2004-II.
Introduction
1) Volume resistivity
Units: Ω ·m. This is the unit cross-sectional area and
Because of eye-opening results in recent years in
unit length resistance value. It is an easy criterion for
research on electrostatic phenomena, there have been
electrostatic leakage. Conductivity (Units S/m) and
effective applications in industry in copiers, dust col-
volume resistivity have reciprocal relationship.
lectors and the like, but there has been no end to the
disasters due to static electricity, such as problems of
2) Conductors and Insulators
damage due to electrostatic discharge in the electron-
These are classified according to volume resistivity as
ics industry where it is often called ESD and fires and
shown in Fig. 1.1) While grounding is effective as a
explosions in the chemical industry. While there is
countermeasure for static electricity with conductors,
specialized knowledge for the prevention of electro-
little or no grounding effect can be expected with insula-
static disasters, it is most important that the operators
tors. Moreover, ungrounded conductors, known as
and staff actually engaging in production in factories
“floating conductors,” are electrostatically very danger-
be intimate with the correct knowledge concerning
ous. The human body can become a floating conductor.
electrostatic safety, discover the potential causes of
electrostatic hazards and consult experts on security
Volume resistivity [Ω · m]
and disaster prevention. This article will be limited to
the prevention of fires and explosions due to static
electricity, and along with introducing discharge phenomena as well as electrostatic charges that people
should know about for preventing electrostatic disasters when working in production sites at factories, we
104
105
Electrical
conductor
106
107
108
109
1010 1011 1012 1013 1014
Moderate
Sufficient effect of
grounding
Insulator
Moderate
No effect of
grounding
will introduce disaster prevention technology and
electrostatic hazard evaluation methods implemented
Fig. 1
Volume resistivity and effectiveness of
grounding
by Sumitomo Chemical.
Explanation of Terms for Static Electricity
3) Leak resistance2)
Leak resistance is the total resistance between an
Here we will give a simple explanation of static electricity terminology as it relates to this article.
SUMITOMO KAGAKU 2004-II
object and the earth, combining the resistance of the
object itself, the contact resistance for electrodes and
1
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
the like, the grounding resistance, etc., and the charge
being electrostatically grounded.
of conductors depends upon leak resistance in general.
Mechanism for Occurrences of Static Electricity and Series of Frictional Electrification
4) Electric potential
Units: V. This is an indirect measure of the size of
1) Mechanism of Occurrence
electrostatic charge.
As is shown in Fig. 2, electrostatic charges occur
through a movement of charges occurring at an inter-
5) Charge
Units: C/g. This expresses the size of the electrosta-
face when two different materials come in contact with
each other, formation of a double electric layer, separa-
tic charge.
tion of charges occurring when the materials are separated and equal and opposite charges arising in the two
6) Electrostatic capacity
Units: F. When the charge of an electrified conduc-
in the proper amounts. The forms of static electricity
tor is Q [C] and the potential V [V], the electrostatic
can be classified according to the way electrification is
capacity C [F] is given by C = Q/V. In addition, the
handled as shown in Fig. 3. With the frictional charg-
energy U [J] stored in the conductor at that time is
ing in (a) in the figure, caution is required because of
1/2 ·C· V 2, 3)
and the ignition hazard can
the risk of fire when a film is impregnated with an
be evaluated by comparing this to the minimum igni-
organic solvent or an organic solvent is being handled
tion energy of combustible materials.
nearby. Another form of frictional charging that can be
given by U =
brought up is charging upon physical discharge
7) Electric field strength
through chute and powder transportation with gas
Units: V/m. If the potential at any two points is
stream. The streaming charging in (b) arises with
given, the electric field strength is obtained by dividing
transport in a pipe, but the probability of fire or explo-
the difference in electric potential by the distance
sion occurring within the pipe where the electrification
between the two points. This is important as an index
occurs is generally low, and the problems usually occur
of whether there is a possibility of a discharge occur-
with explosions in the gas phase part in the tank that is
ring or not.
the destination of the fluid. Streaming charging also
occurs in agitation tanks, but when slurries or two layer
8) Dielectric constant
liquid with liquid arrangements are agitated in agitation
Units: F/m. This is the ε of the relationship between
tanks, caution is required because of the possibility of
dielectric displacement D and electric field E, D = ε E.
sedimentation electrification, which will be discussed
This is a physical property necessary for the electric
later, when the agitation is stopped being much greater
field calculations introduced in this article.
than streaming charging during agitation. Actual
examples of the spray electrification in (c) that can be
9) Electrostatic induction
brought up include cleaning the inside of tanks using a
As is shown in Fig. 3 ( j ), when there is an insulated
shower ball, spray-jet cleaning using water or seawater,
conductor close to a charged body, a charge separation
steam jets, high velocity gas jets containing drops or
occurs on the surface of the conductor because of the
fumes from a venturi scrubber and leaks from flanges
electrostatic induction from the charged body. This is
in high pressure pipes, but the point that must be kept
called electrostatic induction.
10) Grounding and bonding
Material A
B
Grounding is an electrical connection with the earth,
and bonding is an electrical connection with a conductor that is electrically connected with the earth. Both
are very important as safety measures for static electricity. A conductor that is bonded to a grounded con-
(a) Transfer of charge (b) Formation of
by contact
SUMITOMO KAGAKU 2004-II
(c) Electrification by
separation
layer
ductor can be seen as being grounded. Conductors
with a leak resistance of 106 Ω or less can be seen as
electric double
Fig. 2
Mechanism of electrostatic charge
2
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
pipe/hose
roll
or liquid-liquid mixture is being agitated and the agita-
film/paper/cloth
tion stops. From the report of the Ministry of Internal
liquid
(a) Frictional charging
(b) Streaming charging
Affairs and Communications, Fire and Disaster Management Agency it is highly possible that the fire in the
naphtha tank that occurred at the Hokkaido oil plant
due to the Tokachi earthquake occurring at sea in 2003
(c) Spray electrification
(d) Peeling electrification
was caused by the chemical foam bubbles disappearing
over time and sedimentation electrification with the
water drops arising when water leaked precipitating in
the naphtha.4) In the induction charging in ( j ), static
(e) Collisional charging
electricity is induced in an ungrounded metal vessel
(f) Electrification during
transferring liquid
receiving a sample from sampling and tools or human
bodies in the vicinity of a highly charged flexible container, and they may be strongly charged. In either
drips
case, caution is necessary because there can easily be a
drip
surfacing
oil
oil
sedimentation
bubble
source of ignition when combustible solvents or gases
are present in the vicinity.
(g) Sedimentation / Surfacing electrification
2) Series of Frictional Electrification
We have already noted that electrostatic charging
(–)
(+)
2 imbalance of charge
(after splitting)
1 balance of charge
(before splitting)
fiber
(+)
metal
(+)
native material
(+)
synthetic resin
(+)
asbestos
(h) Splitting charging
hair/fur
outside
fragment (+)
outside (+)
glass
mica
wool
nylon
rayon
Pb
water
ice
1 transfer of charge
during freezing
silk
inside fragment (–)
inside (–)
2 internal separation
of charge by
complete freezing
cotton
3 electrification of
each fragment by
complete freezing
wood
human skin
( i ) Electrification when freezing / breaking
electric field
electric conductor
surface induced charge
raw cotton
hemp
glass fiber
Zn
acetate fiber
Al
paper
Cr
ebonite
insulator
Fe
Cu
Ni
( j ) Induction charging
Fig. 3
Example of electrostatic charge
Au
rubber
polystyrene
vinylon
polypropylene
Pt
polyester
acrylic fiber
polyethylene
in mind is that electricity passes through easily, and
polyvinylidene chloride
celluloid
cellophane
there is danger even with water and steam which are
vinyl chloride
thought to be electrostatically safe. Besides the example shown in Fig. 3 (g) for sedimentation electrification,
there is a danger of electrification with precipitation of
solids or liquid-liquid separation arising when a slurry
SUMITOMO KAGAKU 2004-II
polytetrafluoroethylene
(–)
Fig. 4
(–)
(–)
(–)
Examples of series of frictional electrification
3
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
arises through the contact and separation of different
electrostatic discharge phenomena as the source of
materials, but there is affinity among materials that
ignition of combustible materials. The discharges take
come in contact with each other in terms of the size of
several forms, and understanding the characteristics of
the charge, and these are brought together in series of
these is important in preventing fires and explosions
frictional electrification. Fig. 4 shows an example of
due to static electricity. Table 1 is a summary of the
series of frictional
electrification.2)
When two materials
in an series of frictional electrification come in contact
types and characteristics of discharges and the ignition
hazards.1), 5) – 8)
and are separated, the upper material is charged with a
positive (+) charge and the lower one with a negative
(–) charge. The charge increases with the distance
Types of Electrostatic Accidents and Countermeasures
between the vertical positions in the electrification
series. The vertical relationships in the series of fric-
When disasters occur the losses are extensive.
tional electrification are effective over the types of
There is not only the obvious losses related to human
materials. We should note here that is the positioning
life, those due to damage to facilities and those accom-
of polytetrafluoroethylene (Teflon) at the maximum for
panying the stoppage of production activities, but also
the negative polarity in the series of frictional electrifi-
the loss of societal trust in the enterprise as in prob-
cation. In chemical plants that handle powders, Teflon
lems in responsibilities for supply, and problems of
is frequently applied to the insides of hoppers and
effects on the area surrounding the factory, so we must
chutes to improve the physical discharge, but since
prevent these at all costs. Fires and explosions are oxi-
there are many cases where there is electrostatic haz-
dation combustion reactions, and it happens only when
ard, so there must be a sufficient investigation into
the three elements for combustion, 1 combustible
whether or not there is a hazard of dust explosion
materials, 2 oxidizer (air or oxygen) and 3 source of
when this is used.
ignition are met. If one of the conditions is missing,
fires and explosions will not happen. Therefore, in
Discharge Types and Limits to Occurrences
safety measures, we think in terms of eliminating these
conditions. In the following we will give a description
Fires and explosions due to static electricity have
Table 1
of accident phenomena according to type and will intro-
Classifications of electrostatic discharge
Types of discharge
Characteristics
Hazards of ignition
Condition for discharge
Corona discharge
It can occur when an electrode with radius
not sufficiently energetic to ignite
potential : more than several
of curvature less than 5mm experiences a
almost materials except sevral materials
kilovolt
strong electric field.
which has very small minimum ignition
It can occur between a conductor with
sufficiently energetic to ignite gases and
potential : more than dozens of
radius of curvature in the range 5 to 50mm
vapors and some dust cloud which has
kilovolt
and either another conductor or a charged
low minimum ignition energy (less than
average of electric filed : more
insulating surface.
several mJ)
than 1✕105 V/m
energy such as H2, CS2.
Brush discharge
Cone discharge
It can occur along the conical surface of
sufficiently energetic to ignite gases and
diameter of powder : around
the powder heap during filling of a large
vapors and some dust cloud which has
1~10mm
silo with powder which has low
low minimum ignition energy (less than
conductivity.
around 10 mJ)
Lightning-like
It can occur in a large container and a long
sufficiently energetic to ignite gases and
average of electric field : more
discharge
spark jumps from the cloud to the
vapors and dust cloud
than 2.7✕105 V/m
grounded container wall.
Spark discharge
Surface discharge
transitional discharge phenomenon which
sufficiently energetic to ignite gases and
electric field : more than
lead to arc or glow discharge.
vapors and dust cloud
3✕106 V/m
It can occur along the surface of thin
sufficiently energetic to ignite gases and
thickness of insulator : less
insulating layer backed by a conductor.
vapors and dust cloud
than 8mm
surface charge : more than
250µC/m2
SUMITOMO KAGAKU 2004-II
4
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
duce safety measures from the standpoint of 1 eliminat-
region, but when air leaks in to equipment or when gas
ing combustible materials and 2 eliminating oxidizer,
with a composition equivalent to point E leaks into the
and finally we will introduce measures from the stand-
air, it is diluted, and the path moves in the direction of
point of eliminating the source of ignition.
the arrow on line 2 in the triangular diagram, entering
the flammable region. The management with exceed
1) Gas and Vapor Explosions
upper flammable limit always accompanies this kind of
The minimum ignition energy for flammable vapors
hazard. Moreover, the explosive region is affected by
or gases is mostly around 0.2mJ, and since those are
temperature and pressure, so care is necessary in the
easily ignited by brush discharge, constructing ignition
handling of data.
source countermeasures by grounding and the like
alone is insufficient, and in safety engineering we must
l.%
0
80
70
60
Me
tha
ne
[vo
40
l.%
5
]
[vo
90
tra
0
0
80
70
60
50
Oxygen [vol.%]
40
30
20
10
100
n
tio
90
tion. This applies in general to many gases. The direc-
Fig. 5
80
cen
100
]
70
30
n
on
nc
20
ge
60
10
tro
ge
D
Lower flammable limit
about 5%
Ni
xy
lim
it
B
about 15%
greatly according to the oxygen-nitrogen concentration of the arrow on line 1 is the line for dilution of
50
lower flammable limit is basically fixed (approximately
go
and 50% nitrogen. From this figure, we see that the
concentrations, but the upper flammable limit is varies
Up 2
pe
rf
lam
m
ab
le
itin
100% methane, and point B is 20% methane, 30% oxygen
Lim
shown in a triangular diagram in Fig. 5.9) Point A is
E
40
ble region of a methane-oxygen-nitrogen system is
30
with a triangular diagram. As an example the flamma-
5% methane) regardless of the oxygen and nitrogen
1
20
ble region. The flammable region is often represented
A
10
being a nitrogen seal) and avoid entering the flamma-
0
bustible materials and oxidizer (a typical example
10
Therefore, we must control the concentrations of com-
Test conditions
room temperature
atmospheric pressure
Upper spread in vertical tube
(diameter: 5cm)
90
assume that sources of ignition are always present.
0
C About 13%
Flammable limits of methane
methane with air, and point C signifies air itself. The
methane concentration for intersection D of line 1 and
the line for the upper flammable limit is the upper flam-
Table 2
Safety margin of oxygen concentration
mable limit of methane in air, and it is approximately
in case of continually monitoring
in case of non-continually
15%. The limiting oxygen concentration line is a line
of oxygen concentration
monitoring of oxygen concentration
for the oxygen concentration that comes in contact
LOC ≥ 5% : LOC–2%
LOC ≥ 5% : LOC ✕ 0.6%
LOC < 5% : LOC ✕ 0.6%
LOC < 5% : LOC ✕ 0.4%
with the flammable region lines, and it can be seen that
the limiting oxygen concentration for methane is
LOC : Limiting oxygen concentration
approximately 13%. If the oxygen concentration is less
than 13%, we do not enter the flammable region no mat-
2) Flash Fires with Flammable Liquids
ter how the methane concentration changes, and if the
Combustion of flammable liquids is not only contact
methane concentration is less than 5%, we do not enter
combustion with the surface of the liquid, but also
the flammable region even if, for example, the oxygen
combustion of the gas phase in the vicinity of the liq-
concentration increases. Moreover, we should take
uid surface where the mixture of vaporized liquid and
account of safety margin concerning concentration of
the atmosphere (normally air) enter the flammable
flammable gases and oxygen for safety. A concentra-
region, and it can be assumed that combustion contin-
tion of 1/4 the lower flammable limit of concentration
ues with a the repetition of combustion through the
for explosions is recommended for combustible gases.
formation of a flammable atmosphere with the
The U.S. National Fire Protection Association (NFPA)
progress of new vaporization of the liquid due to the
standards for oxygen concentration are given in Table
heat of combustion. Therefore, the minimum ignition
2.10)
Methane concentration of Point E exceeds upper
energy for flammable liquids is the same as that for
flammable limit, and does not enter the flammable
flammable gases, and we can assume that there is
SUMITOMO KAGAKU 2004-II
5
nothing strange about the existence of sources of ignition due to static electricity. However, when it is vapor
from flammable liquids, it is different from methane
and other gases that are normally in a gaseous state,
and temperature at which the vapor pressure of a flammable liquid is equivalent to the lower flammable limit
can be used as an index for safety management. The
temperature at which the vapor pressure of a flammable liquid is equivalent to the lower flammable limit is
defined as the lower flash point, and the temperature
with equivalency to the upper flammable limit is
Lower flash point — flash point by JIS method [°C]
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
: flammable solids
Each data was evaluated by
Tag closed cup method
+2
0
–2
–4
–6
–8
–10
–12
–14
–16
–18
Tag closed Cup
–20
0
20
40
60
Pensky Martens closed Cup
80
100
120
140
160
180 200
Flash point (JIS closed cup method) [°C]
Fig. 6
defined as the upper flash point. At temperatures
Relations between flash point determined
by the JIS closed cup and lower flash point
lower than the lower flash point, the vapor concentration in the gaseous phase is normally below the lower
must be considered in operations where fine particles
flammable, so there is no hazard of ignition. More-
are introduced into organic solvents held at tempera-
over, we must be careful when there is a comparative-
tures below the flash point.12)
ly strong ignition source because there may be temperature rises in places even if the liquid temperature
3) Dust Explosions
is below the flash point, causing flash fire. In addition,
In general the minimum ignition energy for dust is
since the methods for measuring flash points deter-
on the order of 10mJ or higher, and it is one decimal
mined by Japanese Industrial Standard (JIS) and used
place higher than the minimum ignition energy
for the determination of class 4 hazardous materials in
(around 0.2mJ) for common flammable gases and
the Japanese Fire Service Law are not measurements
vapors. Therefore, there are cases where it can be han-
under conditions reaching the ideal liquid-gas equilib-
dled in the air when grounding and other electrostatic
rium, caution must be taken in that there is a differ-
measures are constructed. The minimum ignition
ence from the true lower flash point. Here we will call
energy is important as an index of whether fine pow-
the flash points obtained by JIS the JIS flash points.
ders should be handled in air. As shown in Fig. 7, the
Fig. 6 shows the difference between the lower flash
minimum ignition energy for dust explosions is found
point and the JIS closed cup flash point.11) From this
as the minimum value for energy when various
figure, it can be seen that it is necessary to have a safe-
changes are made to the ignition energy with a fixed
ty margin of about 5°C with a JIS flash point of 20°C
dust concentration, at least three concentrations are
and a margin of about 10°C at one of 80°C. Moreover,
used in operations finding the energies where dust
it is essentially difficult to call maintaining a tempera-
explosions occur and energies where they do not
ture above the upper flash point a safety measure. For
occur and a convex curve is drawn under the bound-
example, it can be assumed that the vapor pressure of
ary between explosions and no explosions. Moreover,
the gasoline in the gas tank of a car is higher than the
dust explosions are greatly affected by the size of the
upper flammable limit, so it is not in the flammable
particles, so evaluations with fine powders are neces-
region, but care must be taken because the gasoline
sary. Up to this point acquisition of data using parti-
vapor is dispersed when filling and creates a flamma-
cles filtered using a sieve with a No. 200 mesh with 75
ble mixture in the vicinity, and there is a possibility
µm openings has been carried out broadly, but the JIS
that discharges from human bodies will ignite it.
Z8818 “Test method for minimum explosible concen-
Moreover, the flash point of kerosene is approximately
tration of combustible dusts” and the ISO6184/1
40°C, and since the vapor pressure is lower than the
“Explosion protection systems-Part1: Determination of
lower flammable limit, it is possible to bring it into use
explosion indices of combustible dusts in air” specify
in everyday life. Moreover, the explosion hazard
the use of powders that have passed through 63 µm
increases when it coexists with combustible dust, and
sieves. 13),
we must be careful that even if the concentration of
affected by the shape of the particles, so when particle
vapor and dust are below their individual lower flam-
shape changes because of changes in production
mable/explosive limit, this is explosive. This point
methods (changes in crystallization methods, for
SUMITOMO KAGAKU 2004-II
14)
Moreover, dust explosiveness is also
6
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
example), examinations of whether the minimum igni-
ual solvent vapor will be ignited by discharge from the
tion energy must be re-evaluated or not should be
charged water mist.
made. With bag filters for collecting fine powders,
The explosion hazards with flammable liquid mists is
comparatively large hoppers and silos and the like, it is
not related to the flash point. The propagation mecha-
common for to be prepared just in case and install
nism for mist explosion can be thought of as being the
explosion relief
vent.15)
same as that for dust explosions. If energy is given to
the mist through a discharge, there are local rises in
temperature, and the generation of dissociated gases
Ignition energy [J]
due to vaporization or thermal decomposition is accelerated, locally forming flammable gas-air mixtures and
MIE
distant (in other words, the mist concentration falls),
burning. The heat radiating from the combustion
heats the adjacent mist, and forms new flammable gasair mixtures to burn, and these are propagated one
after another. When the spaces in the mist become too
the propagation cycle is interrupted, and the mist
explosion stops. This limiting mist concentration is
Dust concentration [g/m3]
Fig. 7
called the lower limit of mist explosion. Fig. 8 is a
Estimating method of minimum ignition
energy
summary of a mist explosion test apparatus developed
by Sumitomo Chemical.17)
Moreover, with air transport and the like, it is sometimes effective to control the dust concentration below
Wire mesh
the lower explosive limit, but it is generally difficult to
control dust concentration when fine particles are intro-
Table 2 can be used as is for controlling oxygen con-
2000mm
gases and vapors, the oxygen concentration index in
600mm
centration.
liquid
Electrodes
800mm
duced in manual operations. Just as with flammable
Cylinder
(clear vinyl
chloride)
Mist receiver
2 Fluid nozzle (not measured)
4) Mist Explosions1), 16)
Wire mesh
With mist explosions, it is often difficult to control
gas
centration is the easiest and most certain safety measure. The minimum ignition energy for mist can be
Balance
Mist receiver
the mist concentration, so controlling the oxygen con-
Fig. 8
Test temp. : about 20°C
Sample : aqueous solution
of organic solvent
(flash point 58°C)
Photo of mist explosion
Example of mist explosion and test apparatus
assumed to be similar to that for flammable gases and
vapors from a safety engineering point of view, and we
should think that it is not strange for electrostatic igni-
The main point in the evaluation of mist explosions is
tion sources to be present at all times. As is shown in
how a mist cloud that explodes easily is formed, and
Fig. 3 (c), static electricity is generated when the mist
the two-fluid nozzle is superior in terms of this. The
is produced. The periphery of the mist is blocked by
mist concentration measurements are measurements
highly insulating gases such as air, so the charge dis-
of the mass after the spray from the nozzle has stopped
persion rate is slow. Therefore, even water and
and the mist, which falls with gravity, has collected,
108Ω · m
and it is found by dividing by the volume of the suspen-
or lower, are strongly electrified. In spray cleaning,
sion space. It is possible for mist explosions to occur
pure water becomes more strongly electrified than
when liquid in spray cleaning of tanks, liquid in mist
highly insulating liquids such as toluene or hexane.
separators or that being transported in pipes splashes,
Therefore, even when residual solvents are cleaned
but the hazard of this is often missed, because the fre-
with a water spray, there is a possibility that the resid-
quency of occurrence is lower than incidences of explo-
methanol, which have a volume resistivity of
SUMITOMO KAGAKU 2004-II
7
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
sions of dust, gases and vapors and fires igniting in
can bring up cases where the body is electrified by tak-
flammable liquids.
ing on this electrostatic induction and cases where,
when, in the operation of putting in or taking out a fine
powder from a flexible container with insulating prop-
5) Fires of Dust Heap
Self-reactive substances such as explosives, and
erties, the flexible container becomes strongly electri-
metallic powders of magnesium, aluminum, tantalum,
fied and the human body takes on the electrostatic
zirconium and the like are often highly sensitive to
induction. Countermeasures we can mention are con-
electrical sparks, and it is possible for dust heap to be
ductivity management of the floor and the wearing of
ignited by electrostatic discharge. The deactivation of
antistatic shoes and antistatic work clothes. We will
self-reactive substances such as explosives using nitro-
introduce an example of evaluating the electrification of
gen seals may have no effect. Some metal dust heap
the human body separately. Moreover, even if electro-
have extremely low minimum ignition energy, but
static countermeasures for the human body were per-
since they undergo oxidative combustion, deactivation
fect, it is still necessary to be careful because it is possi-
with nitrogen is effective. See the
references 1)
for
details on the hazards of fire with dust heap.
ble to receive an electric shock from other electrified
conductors and insulators. If there is a location or
operation where one receives even a little shock within
Measures for Eliminating Ignition Sources
a chemical plant, the cause must be pursued and suitable countermeasures constructed.
There are many types of measures for eliminating
ignition sources, and their use must be divided accord-
2) Prevention of Metal Electrification
ing to the situation. In addition, since there are cases
Grounding or bonding is carried out. Care must be
where an error in the countermeasure procedure
taken that bonding has no effect unless done with a
means the effect will not be obtained and may have the
grounded conductor.
reverse effect, so it is preferable that safety measures
be determined in consultation with experts in static
3) Prevention of Electrification of Insulating
electricity. Given the limitations of this article, the fol-
Materials
lowing is a simple introduction, and one should see the
Since this is difficult, it is smart to reduce the use in
references introduced at the end of this article and get
chemical plants as much as possible. When use is
guidance for the details.1), 2), 8), 10), 18)
unavoidable, expert evaluations should be requested or
nitrogen seals used.
1) Prevention of Electric Shock
There is not only the possibility of grave accidents
4) Prevention of Electrification of Liquids During
occurring with falling or the tendency to fall due to
Pipe Transport
receiving an electric shock, but also, in locations where
The initial flow limit (1m/sec or less) and flow limit
flammable gases or explosive dust are present, there is
for a static state should be adhered to.2) Relaxation
the possibility that being shocked creates an ignition
pipe should be installed as necessary.
source that could cause a fire or explosion. Since the
human body is a conductor, it can be assumed that the
5) Handling of Flammable Liquids in Manual
almost all of charge of human body is released when
Operations
discharge happened. For example, if a human body
Electrostatic countermeasures for the human body
with an electrostatic capacity of 100pF is electrified to
should be worked out. After a container is transported,
2kV, the stored energy is 0.2mJ, and this reaches the
relaxation time should be taken. Grounding and bond-
minimum ignition energy for common flammable gases
ing of metal containers should be done from before
and vapors. If the electrostatic measures taken for the
work starts to until it is completed. Care must be taken
human body are insufficient, this degree of electrifica-
that drum transporters and other mobile equipment
tion can easily arise. In terms of the electrification of
does not become floating conductors. Filter cloth with
the human body, the bottom of shoes that have insulat-
highly insulating properties should not be used in open
ing properties are electrified by friction electrification
systems. Funnels, pumps and the like with insulating
between the floor and the shoes when walking, and we
properties should not be used in the handling of flam-
SUMITOMO KAGAKU 2004-II
8
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
mable liquids with conductivities of 108S/m or less.
ening-like discharge occurring can be examined. In
Other equipment matched to the work should be used
addition, by finding the energy from Equation (2) and
correctly.
comparing it with the minimum ignition energy it is
possible to estimate whether the discharge would have
ignition capacity. The space charge density in Equa-
6) Hand Loading of Fine Powders18)
Caution should be taken about electrostatic charging
tion (1) is found from the product of the electrification
when fine powders are dusted off of containers and
charge and the dust concentration. For example, we
bags. Deactivation measures should be used when fine
can consider the powder dryer in Fig. 10. The space
powders are introduced into flammable liquids unless
charge density is found at the point of measurement by
agreement can be obtained from safety experts. The
using an aspiration type Faraday Cage as is shown in
possibilities for changing the order of loading (loading
detail in Fig. 11.19) Fig. 12 is an example of the calcu-
the fine powder first) should be investigated.
Faraday Cup
(aspiration type)
7) Use of Antistatic Agents
There are ones that are kneaded in and ones that are
Dryer
applied to insulating materials. There are antistatic
agents that exhibit effects when added to liquids in
amounts on the order of ppm. However, there is a
need to examine problems with product quality.
4000mm
Dry air
Thick paper
Wire mesh
Electric field
meter
Probe
8) Use of Static Charge Eliminators
Experts in the use of static charge eliminators
Recorder
Coaxial cable
Purge
Shield
Flow meter
Insulator
Pump
should be consulted, and their use should be determined with sufficient examination of whether the static
Fig. 10
eliminator itself is a possible source of ignition.
Measurement of charged density of dust
cloud
Example of Evaluation
1) Evaluation of Electrostatic Hazard of Dust
Cloud Using Electric Field Simulation
Insulator
The hazard of ignition due to self-discharge from a
dust cloud or mist cloud can be evaluated using electric
field simulations. In electric field calculations, the electric potential and the electric field strength, which is its
Filter paper
Wire mesh
slope, can be found by solving Equation (1) shown in
Fig. 9, and the possibilities of brush discharge or light-
Fig. 11
Coaxial cable
Details of faraday cup (aspiration type)
Poisson’s equation
∂2φ
∂ 2φ
∂ 2φ
+
+
= –
2
2
∂x
∂y
∂z2
ρ
ε
(1)
Energy calculation
U=
1
2
Shape of paddle
∫ ε · E2 dV
φ : Potential [V]
(2)
ρ : Space charge density [C/m3]
ε : Dielectric constant [F/m] U : Energy [J]
E : Electric field strength [V/m]
Fig. 9
Basic formula of electric fields
SUMITOMO KAGAKU 2004-II
Fig. 12
Simulation results of electric field strength
9
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
lated results for electric field strength inside the dryer
fine powders that are strongly sensitivity causing,
shown in Fig. 10 using ElecNet software from the
there are cases where a need arises for wearing spe-
Canadian company Infolytica Corp. (agent: Advanced
cial work clothes that are not normally used. In this
Technologies Co., Ltd.). As the color moves from dark
instance, the work clothes must not simply be select-
blue through yellow to red, the electric field strength
ed for the safety and hygiene of the human body, but
becomes stronger. From the simulation, we can see
also the fire and explosion prevention aspects must be
that the electric field strength is high at the edges of
considered in the determination of work clothes and
the rotating paddles that play a role in crowding the
working methods. Here we will take up an example of
powder. The form of discharge for which there is a pos-
polypropylene working clothes and introduce the
sibility of igniting the dust cloud itself due to self-dis-
results of an examination of the relationship between
charge from the dust cloud is a lightening-like dis-
the presence and absence of electrostatic countermea-
charge (see Table 1), and from the results of the simu-
sures for the floors and human body and the electro-
lation, it should be judged that there is a possibility of a
static hazards. Polypropylene working clothes are
lightening-like discharge in areas above the average
actually sold, and the volume resistivity is 1.2 x 1015
V/m, including areas
Ω · m and the surface resistivity 2.1 x 1016 Ω, so it con-
of 3 x 106 V/m or more, which have an electric field
sists of highly insulating materials. As a result of
strength that would locally break down the insulation
measurements with a voltmeter (Kasuga Denki, Inc.
of air. In the case of mist explosions, the minimum igni-
KS-533) of the surface potential of polypropylene work
tion energy is equivalent to that for flammable gases,
clothes when there has been friction with a synthetic
and there is a possibility of ignition due to brush dis-
fiber vest several times in a test room with 50% humid-
electric field strength of 2.7 x
105
charge. It can be assumed that brush discharge can
ity and a temperature of 20°C, where a conductive
occur in the area of 1.0 x 105 V/m or higher. If the lat-
plate is laid out on urethane flooring material with a
est computer simulations are used, it is possible to cal-
leak resistance of 9 x 107 Ω, and this is stood on with
culate the sum of the energy for the continuous area
antistatic shoes and antistatic clothes and further
where this electric field strength exceeds a fixed
when polypropylene work clothes are worn, it was
threshold value, and by comparing this to the mini-
found that the polypropylene work clothes had a sur-
mum ignition energy, it is possible to examine whether
face potential of approximately 20 kV. This is a result
ignition would happen or not when an actual discharge
that greatly exceeds the maximum value of 10 kV for
occurs. This method has problems such as the deter-
the electrification index2) for insulators given in Table
mination of the space charge density being compara-
3. In addition, the electric potential of the human
tively difficult and the effort for modeling of complex
body was measured in the same manner using a Fuji-
shapes, but it is an important evaluation method for
maru SR-111 Electrostatic Voltmeter, and the electro-
safety. Furthermore, it does not end with calculations
static capacity was measured using a Hewlett Packard
of electrostatic fields, and research into dynamic simu-
4332A LCR Meter; the results of calculations of the
lations of electric fields formed by charged particle
energy stored in the human body are given in Table
groups has been introduced at academic society meet-
4. It was clear that in cases 3 – 6, the human body
ings; this is a field in which we can expect to see tech-
could store a level of energy that would be able to
nical progress in the
future.20)
ignite common organic solvents. What should be
noted here is that, as is demonstrated in case 6, even
2) Evaluation of Electrostatic Hazard of Human
Body
if polypropylene work clothes are not worn, safety is
not secure if the shoes are not of an antistatic materi-
As general measures for preventing human body
electrification in chemical plants, 1 assurance of conductivity in the floor surfaces according to the sub-
Table 3
Recommendations (potential of insulator)
stances handled and the working environment, 2 the
wearing of antistatic shoes and 3 the wearing of anti-
Minimum ignition energy [mJ]
Indices of potential [kV]
< 0.1
<1
static work clothes are used. However, to work out
0.1 ~ 1
<5
measures for preventing exposure due to the han-
1 ~ 10
< 10
dling of materials in filling and introduction work with
> 10
< 10
SUMITOMO KAGAKU 2004-II
10
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
Table 4
Results (human charge)
Items
Floor
Case 1
Case 2
polyurethane floor +
polyurethane floor
conductive grounded plate
Shoes
Clothes
Case 3
Case 4
Case 5
Case 6
polyurethane floor +
polyurethane floor +
polyurethane floor
polyurethane floor
conductive grounded plate
conductive grounded plate
antistatic shoes
antistatic shoes
antistatic shoes
slippers
slippers
slippers
antistatic working
antistatic working
polypropylene
polypropylene
polypropylene
antistatic working
wares
wares
working wares
working wares
working wares
wares
70
420
940
6100
7420
7970
120
98
120
64
60
60
2.9E–04
8.6E–03
0.1
1.2
1.7
1.9
Maximum potential [V]
Electrostatic capacity of
human body [pF]
Energy accumulated to
human body [mJ]
al, even if antistatic clothes is worn. Moreover, anti-
tion against Hazards arising out of Static Electrici-
static shoes and clothes may have a great difference
ty in General Industries, (1988), TECHNOLOGY
concerning the ability for eliminating electrostatic
INSTITUTION OF INDUSTRIAL SAFETY
hazards, so it is recommended that performance be
3) Heinz Haase Translated by M. Wald: Electrostat-
confirmed on an individual wares when these are
ic Hazards Their Evaluation and control, (1977),
employed.
VERLAG CHEMIE
4) SANGYOU TO HOAN-Dai 20 Kan Dai 24 Gou
(Japanese), (2004)
Conclusion
5) S. Masuda: SAIKIN NO SEIDENKI KOUGAKU,
There have been sudden occurrences of electrostatic
disasters at plants that have worked continuously for
(1974), THE HIGH PRESSURE GAS SAFETY
INSTITUTE OF JAPAN
more than ten years without an accident and in opera-
6) Jones T.B. and King J.L.: Powder Handling and
tions that have been performed by hand by humans
Electrostatics: Understanding and Preventing
several thousand times without accidents, so past histo-
Hazards, (1991), LEWIS
ry gives no guarantee to the future.
It is important that we know electrostatic potential
hazards of various operations by studying electrostatic
prevention
recommendations2)
or case studies of elec-
7) Glor M.: Electrostatic Hazards in Powder Handling, (1988), JHON WILEY & SONS INC.
8) NFPA: NFPA77 Recommended Practice on Static
Electricity, (2000)
trostatic incident and each worker in chemical plant
9) S. Yagyu: GASU OYOBI JYOKI NO BAKUHATU-
discovers electrostatic potential hazards which lurking
GENKAI, 50 (1977), JAPAN SOCIETY FOR SAFE-
in his operations. Next, the electrostatic hazards that
TY ENGINEERING
are discovered should not be left as is, and the hazards
should be reduced by referring to recommendations
10) NFPA: NFPA69 Standard on Explosion Prevention Systems, 7 (1997)
and handbooks and, when necessary, consulting
11) S. Yagyu: Safety Document of the Research Insti-
experts about countermeasures. This is connected to
tute of Industrial Safety RIIS-SD-86 Diagrams Relat-
improving safety records. We will be glad if this article
ed to Flashing Temperatures and Flammability
is of some benefit to the reader in that sense.
Limits (1), (1986), JAPAN SOCIETY FOR SAFETY ENGINEERING
References
12) Rolf K. Eckhoff: Dust Explosions in the Process
Industries, second edition, (1997), BUTTER-
1) THE INSTITUTE OF ELECTROSTATICS JAPAN:
SHINBAN SEIDENKI HANDOBUKKU, (1998),
OHMSHA, Ltd.
2) MINISTRY OF LABOUR RESEARCH INSTI-
WORTH HEINEMANN
13) JAPANESE STANDARDS ASSOCIATION, JIS Z
8818 Test method for minimum explosible concentration of combustible dusts, (2002)
TUTE OF INDUSTRIAL SAFETY JAPZN: RIIS-
14) ISO 6184/1 Explosion protection systems–Part1:
TR-87-1 RECOMMENDED PRACTICE for Protec-
Determination of explosion indices of combustible
SUMITOMO KAGAKU 2004-II
11
Evaluation and Prevention of Electrostatic Hazards in Chemical Plants
dusts in air, (1985)
15) John Barton: DUST EXPLOSION PREVENTION
AND PROTECTION, (2002), IChemE
TUTE OF INDUSTRIAL SAFETY JAPZN: RIISTR-85-3 A supplement to “Recommended Practice
for Protection against Hazards arising out of Static
16) Daniel A. Crowl and Joseph F. Louvar: Chemical
Electricity in General Industries” –Applications of
Process Safety: Fundamentals with Applications,
Safety Measures to Selected Production Facilities,
(1990), PRENTICE HALL
Work, etc.,(1986), TECHNOLOGY INSTITUTION
17) K. Ota et al.: DAI 34 KAI ANZEN KOUGAKU
KENKYUU HAPPYOU KAI KOEN YOKOSHUU
(JAPANESE), 59 (2001)
18) MINISTRY OF LABOUR RESEARCH INSTI-
OF INDUSTRIAL SAFETY
19) Y. Kumagae et al.: SUMITOMO KAGAKU,1990II, 122 (1990)
20) A.Ohsawa, Powder Tech,135–136, 216 (2003)
PROFILE
Kiyoshi O TA
Sumitomo Chemical Co., Ltd.
Process & Production Technology Center
Research Associate
SUMITOMO KAGAKU 2004-II
12